This invention relates to positioning apparatus, and in particular to positioning apparatus suitable for determining the position of a moving vehicle, and to a positioning method.
Tracking of the position of high dynamic moving vehicles whilst in flight is desirable for many reasons, particularly as it can allow the location of a vehicle to be accurately determined even though the location may not be visible from a viewing site.
For this purpose, it is known to use a translator device forming part of the vehicle, and a base or ground station remote from the vehicle. The translator device receives positioning signals from a number of satellites of, for example, the GPS constellation, and amplifies and frequency translates the signals before transmitting them as translator signals to the base station. A positioning receiver in the base station then processes the translator signals to determine position information relating to the translator device and thus the host vehicle. By recording the translator signals, the base station can plot the path of the vehicle accurately and with whatever time intervals are required, after the vehicle""s flight. Translator systems are resistant to the effects of acceleration bursts. However, the bandwidth required for Inclusion of simplified GPS receivers in wireless telephones and other mobile devices with communication capability has been proposed. Such devices may include personal digital assistants (PDAs), laptop computers and in-car navigation systems. The GPS receivers comprise circuitry arranged to process positioning signals transmitted by the satellites, to determine interim position information. Extraction of the data carried on the positioning signals is carried out at a base or ground station, which can thereby calculate position information relating to the GPS receiver and thus the wireless telephone.
Such a system is beneficial in that it removes a substantial processing burden from the mobile device, yet allows rapid determination of its position. Furthermore, the accuracy of location determination of such a system can easily be increased through the use of differential GPS techniques.
According to a first aspect of this invention, there is provided positioning apparatus for use in a moving vehicle comprising: a receiver for receiving coded ranging signals from each of a plurality of remote transmitters including at least one positioning satellite; a code correlator device arranged to correlate a locally provided code with the code contained in modulation of the coded ranging signals, and to provide correlator output signals in response thereto; a code tracking device responsive to at least some of the correlator output signals to provide code tracking output signals; a carrier frequency estimator device, arranged to receive at least some of the correlator output signals, to estimate the carrier frequency of the coded ranging signals, and to provide carrier frequency estimation signals in response thereto, the carrier frequency estimator device being arranged to operate in two modes, the sampling rate of the correlated output signals being higher in an initial mode than in a subsequent mode; a carrier sampling device arranged to receive and to sample at least some of the correlator output signals to provide carrier sample signals from which information about the phase of a carrier of the coded ranging signals can be determined; and a transmitter arranged to receive the code tracking output signals, the carrier sample signals and the carrier frequency estimation signals, and to transmit signals which are representative of the code tracking output signals, the carrier sample signals and the carrier frequency estimation signals, to allow position information relating to the apparatus to be determined by further processing at a location remote from the vehicle.
In this way, it is possible to construct positioning apparatus which is capable of providing precision position tracking and which may better maintain satellite carrier tracking lock, especially where high values of acceleration and jerk may be experienced. In addition, the bandwidth required for transmitting the relevant signals to a ground or other remotely located base station may be considerably less than that required for the translator system mentioned above.
Preferably, the carrier frequency estimator device is an open loop device, Alternatively, it may comprise a frequency locked loop, which may allow reduced data rates and therefore a reduced bandwidth channel required for transmission between the positioning apparatus and a ground station.
The remote transmitters typically consist of positioning satellites, such as those forming the global positioning system (GPS) constellation. However, the remote transmitters could be ground-based pseudo-lites, or a mix of pseudo-lites and satellites. A pseudo-lite is a device which transmits positioning/timing signals at the same frequency as positioning satellites although, in the case of GPS pseudo-lites, the modulating code is not the same as the code transmitted by any of the satellites but it is of the same class. Pseudo-lites may transmit positioning/timing signals at any frequency. Those signals are synchronised with GPS. For GPS signals, the modulating code is an N=10 Gold code. The term for positioning satellites used in this specification is space vehicles (SVs).
In accordance with a second aspect of the invention, there is provided a method of determining position information relating to a vehicle, the method comprising: in a receiver associated with the vehicle: receiving coded ranging signals from each of a plurality of remote transmitters including at least one positioning satellite; correlating a locally provided code with the code contained in modulation of the coded ranging signals, and providing correlated output signals in response thereto; tracking at least some of the correlated output signals, and providing code tracking output signals in response thereto; initially estimating the frequency of the carrier of the coded ranging signals from samples of the correlated output signals, subsequently controlling an oscillator of the receiver to adopt a frequency substantially corresponding to the estimated frequency, subsequently estimating the frequency of the carrier of the coded ranging signals from samples of the correlated output signals having a lower sampling rate that the samples used in the initial estimation step and providing carrier frequency estimation signals in response thereto; sampling at least some of the correlated output signals to provide carrier sample signals from which information about the phase of the carrier of the coded ranging signals can be determined; and transmitting signals representative of the code tracking output signals, the carrier sample signals and the carrier frequency estimation signals; and in a remote station, processing the transmitted signals to determine position information relating to the vehicle.
In accordance with a third aspect of the invention, there is provided apparatus for calculating the position of a remote vehicle mounted receiver capable of receiving coded ranging signals from each of a plurality of remote transmitters, the apparatus comprising: a wireless data receiver; a carrier loop reconstructor device arranged to receive carrier frequency estimation signals and carrier sample signals received at the wireless data receiver, to detect the phase and the frequency of carrier signals received at the remote receiver on the basis thereof, to reconstruct the carrier of the ranging signals as experienced by the vehicle-mounted receiver, and to provide carrier output signals in response thereto; a pseudo-range measurement device arranged to receive code tracking signals received at the wireless data receiver, to determine positioning information relating to the remote receiver on the basis thereof, and to provide positioning signals in response thereto; and a location calculator arranged to calculate a location of the remote receiver on the basis of the carrier output signals and the positioning signals.
In accordance with a fourth aspect of the invention, there is provided a complex Costas loop arrangement, comprising: a complex frequency changer, having a real and imaginary digital signal inputs and real and imaginary oscillator digital inputs, the complex frequency changer being arranged to provide signals at real and imaginary digital outputs having a frequency dependent on the frequencies of signals received at the inputs; and an oscillator having an input connected, preferably via a device having a transfer function, to the outputs of the frequency changer device; the oscillator being arranged to provide oscillator signals to the oscillator inputs of the complex frequency changer dependent on signals received at its input.